(1) Field of the Invention
The present invention is directed to a gain variation compensating circuit for maintaining the linearity of a feedforward linear amplifier when changes in gain of a power bank amplifier or amplifiers vary due to aging or failure conditions. In particular, the present invention compensates for gain variations thus restoring balance and fundamental signal cancellation of the feedforward amplifiers thereby retaining good linearity characteristics of the amplifiers.
(2) Description of Related Art
Feedforward cancellation amplifiers subtract matched pairs of signals for cancellation purposes. Each stage of cancellation has two such cancelers; one for cancelling fundamental signals to obtain distortion products; and the other for cancelling distortion products in the output. Of these two cancellation processes, the most difficult to match and balance is the cancellation of the fundamental signals. Cancellation of fundamental signals is important because loss of balance in fundamental signal cancellation leads to an increase in fundamental signal residue. This in turn drives the distortion cancelling amplifier harder and results in more distortion being generated by the distortion cancelling amplifier. This type of distortion is then directly coupled and added to the output power of the feedforward amplifier. An increase in distortion power in the output of amplifier is equivalent to a reduction in linearity.
Cancellation of fundamental signals is difficult because the cancellation loop includes a power bank which generates the raw power of the amplifier. Matching pairs of signals for cancellation purposes must be done in both amplitude and phase. For high frequency amplifiers covering a frequency band of 2 to 30 MHz, phase matching is straightforward. Since the wavelengths involved are greater than 10 meters, phase matching is stable and varies little with varying conditions. However, amplitude balance is more difficult for the reason that the gain of the amplifiers (such as the power bank) in the loop will vary with varying conditions. In addition, failures in the amplifiers cause reduction in gain, thus upsetting the balance of the amplifier. In particular, when the power bank (main amplifier) fails, the effects of gain variation in the circuit upsets the balance in the circuit.